RFR: 8257831: Suspend with handshakes [v2]

[Richard Reingruber]

On Tue, 6 Apr 2021 11:56:36 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> 
> 
> I do not follow, the OM is unrelated.
> The suspender do not hold the OM.
> 
> What happens is:
> 
>     * Thread A wait on OM X in blocked.
> 
>     * Thread Z suspends Thread A, thread Z have no relation to OM X.
> 
>     * Thread B unlocks OM X, thread B have no relation to the suspend request.
> 
>     * Thread A locks OM X while blocked.
> 
>     * Thread A was not allowed to lock OM X due to it is actually suspended.
> 
>     * Thread A unlocks OM X and waits until resumed.
> 
> 
> So while A and B fight over OM X, A is trying to figure out if Z suspended him while grabbing the lock.

If understand the example correctly then the suspend operation and the
operations on OM X are unordered with respect to each other. If so, then we can
put them into an order where the suspend happens after "Thread A locks OM X while
blocked.". We are free to do this; there's no synchronization that would prevent it.

Also when the suspend request happened while A was blocked then after
`current->set_thread_state_fence(_thread_blocked_trans);` the check of
`is_suspended()` will return true even if the handshake state lock is not
acquired for the check. And if Z tried to suspend A after the state change then
Z will block because A is not safe anymore.

Sorry for insisting. I really hope I'm not wrong ;)

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PR: https://git.openjdk.java.net/jdk/pull/3191